Lecture 2
Physical Layer Standards
DTE-DCE INTERFACE
• Two terms are important in computer networking:
– Data terminal equipment(DTE)
– Data circuit-terminating equipment(DCE)
• There are usually four basic functional units involved in
the communication of data; A DTE and DCE on one end
and DTE on the other end. As shown in figure.
• The DTE generates the data and passes them, along
with any necessary control characters, to a DCE. The
DCE converts the signal to a format appropriate to the
transmission medium and introduces it onto the
network link.
• When the signal arrives at the receiving end, this
process is reversed.
Data terminal equipment (DTE)
• Data terminal equipment (DTE) includes any unit
that functions either as a source of or as a
destination for binary digital data.
• At the physical layer, it can be a terminal,
microcomputer, computer, printer, fax machine, or
any other device that generates or consumes digital
data.
• DTEs do not often communicate directly with one
another; they generate and consume information
but need an intermediary to be able to
communicate.
Data circuit-terminating equipment (DCE)
• Data circuit-terminating equipment (DCE) includes any
functional unit that transmits or receives data in the form of
an analog or digital signal through a network.
• At the physical layer, a DCE takes data generated by a DTE,
converts them to an appropriate signal, and then introduces
the signal onto the telecommunication link. Commonly used
DCEs at this layer include modems (modulator/
demodulators).
• In any network, a DTE generates digital data and passes
them to a DCE; the DCE converts the data to a form
acceptable to the transmission medium and sends the
converted signal to another DCE on the network. The second
DCE takes the signal off the line, converts it to a form usable
by its DTE, and delivers it.
• A DCE is any device that transmits or receives data in the
form of an analog or digital signal through a network.
DTE-DCE interface
Comput
er
Terminal
, fax,
printer
etc.
Modem
etc.
Over the years, many standards have been
developed to define the connection between a DTE
and a DCE. Through this solution differ, each
standard provides a model for the mechanical,
electrical, and functional characteristics of the
connection.
Physical Layer Standards
• Electronic Industries Association (EIA) and
• International Telecommunication UnionTelecommunication Standards Committee
(ITU-T).
• The EIA standards are called, appropriately
enough, EIA-232, EIA-442, EIA-449, and so on.
• The ITU-T standards are called the V series
and X series.
EIA-232 Interface
• EIA-232 is one important interface standard developed
by the EIA, which defines the following characteristics
of the interface between DTE and DCE:
• Mechanical
• Electrical and
• Functional
• Originally issued in 1962 as the RS-232 standard
(recommended standard), the EIA-232 has been
revised several times.
• The most recent version, EIA-232-D, defines not only
the type of the connector to be used but also the
specific cable and plugs and the functionality of each
pin.
Mechanical Specifications
• The mechanical specification of the EIA-232
standard defines the interface as a 25-wire cable
with a male and a female DB-25 pin connector
attached to either end. The length of the cable
may not exceed 15 meters.
• A DB-25 connector is a plug with 25 pins or
receptacles, each of which is attached to a single
wire with a specific function. With this design,
the EIA has created the possibility of 25 separate
interactions between a DTE and DCE.
• Fewer are actually used in the current practice,
but the standard allows for future inclusion of
functionality.
• The EIA-232 calls for a 25-wire cable
terminated at one end by a male connector
and the other end by a female connector.
• The term male connector refers to a plug with
each wire in the cable connecting to a pin.
• The term female connector refers to a
receptacle with each wire in the cable
connecting to a metal tube, or sheath.
• In the DB-25 connector, these pins and tubes
are arranged in two rows, with 13 on the top
and 12 on the bottom.
Electrical Specification
• The electrical specification of the standard
defines the voltage levels and the type of
signal to be transmitted in either direction
between the DTE and the DCE.
Sending the Data
• The electrical specification for sending data is
shown in next figure. EIA-232 states that all the
data must be transmitted as logical 1s and 0s
(called mark and space) using NRZ-L encoding,
with 0 defined as a positive voltage and 1 defined
as a negative voltage.
• However, rather than defining a single range
bounded by highest and lowest amplitudes, EIA232 defines two distinct ranges, one for positive
voltage and one for the negative voltage.
• To be recognized as data, the amplitude of a
signal must fall between 3 and 15 volts or
between -3 and -15 volts, by allowing valid signals
to fall within two 12-volt ranges.
Sending Data
Control and Timing
• Only 4 wires out of the 25 available in an EIA-232 interface are used
for data functions.
• The remaining 21 are reserved for function like control, timing,
grounding, and testing.
• The electrical specifications for these other wires are similar to those
governing data transmission, but simpler. Any of the other functions
is considered ON if it transmits a voltage of at least +3 and OFF if it
transmits a voltage with a value less than -3 volts.
• Figure given next shows one of these signals. The specification for
control signals is conceptually reversed from that for data
transmission. A positive voltage means ON and a negative voltage
means OFF. Also note that OFF is still signified by the transmission of
a specific voltage range.
• A final important function of the electrical specification is the
definition of bit rate, EIA-232 allows for a maximum bit rate of 20
Kbps, although in practice this often is exceeded.
Control
Functional specification
• Two different implementations of EIA-232 are
available: DB-25 and DB-9.
DB-25 Implementation
• EIA-232 defines the function assigned to each of the 25
pins in the DB-25 connector. Figure shows the ordering
and functionality of each pin of a male connector.
• Remember that a female connector will be the mirror
image of the male, so that pin 1 in the plug matches
tube 1 in the receptacle, and so on.
• Each communication function has a mirror or
answering function for traffic in the opposite direction,
to allow for full-duplex operation.
• For example, pin 2 is for transmitting data, while pin 3
is for receiving data. In this way, both parties can
transmit data at the same time.
EIA-232
Data Pins
Control Pins
Timing Pins
DB-9 implementation
• Many of the pins on the DB-25
implementation are not necessary in a single
asynchronous connection. Note that there is
no pin-to-pin relationship in the two
implementations.
• An example
• Step1: shows the preparation of the interface for transmission. The
two grounding circuits, 1(shield) and 7 (signal ground), are active
between both the sending computer/modem combination (left)
and the receiving computer/modem combination (right).
• Step 2: This step ensures that all four devices ready for
transmission. First the sending DTE activities pin 20 and sends a DTE
ready message to its DCE. The DCE answers by activating pin 6 and
returning a DCE ready message. This same sequence is performed
by the remote computer and modem.
• Step 3: this step set up the physical connection between the
sending and receiving modems. This step can be thought of as the
ON switch for transmission. It is the first step that involves
• the network. First, the sending DTE activities pin 4 and sends its DCE a
request-to-send message. The DCE transmits a carrier signal to the idle
receiving modem. When the receiving modem detects the carrier signal, it
activates pin 8, the received line signal detector, telling its computer that a
transmission is about to begin. After transmitting the carrier signal, the
sending DCE activates pin 5, sending its DTE a clear-to-send message. The
remote computer and modem perform the same step.
• Step 4: is the data transfer procedure. The initiating computer transfers its
data stream to its modem over circuit 2, accompanied by the timing pulse
of circuit 24. The modem converts the digital data to an analog signal and
sends it out over the network. The responding modem retrieves the signal,
converts it back into digital data and passes the data along to its computer
via circuit 3, accompanied by the timing pulse of circuit 17.
• Once both sides have completed their transmissions, both computers
deactivate their request-to-send circuits; the modems turn off their carrier
signals, their received line signal detectors (there is no longer any signal to
detect), and their clear- to-send circuits (step 5).
Synchronous Full-Duplex Transmission
Synchronous Full-Duplex Transmission
Animation of EIA-232
• http://www.mhhe.com/engcs/compsci/forouz
an/dcn/graphics/animations/06_12.swf
Null modem
• It is used when we need to connect two DTEs in the
same building, for example, two workstations or a
terminal to a workstation.
• Modems are not needed to connect two compatible
digital devices directly; the transmission never needs to
cross analog lines, such as telephone line, and
therefore does not need to be modulated.
• But you do need an interface to handle the exchange
(readiness establishment, data transfer, receipt, etc.),
just as an EIA-232 DTE-DCE cable does.
• The solution, provided by the EIA standard, is called a
null modem. A null modem provides the DTE-DTE
interface without the DCEs.
Crossing connections in Null Modems
• For transmission to occur, the wire must be crossed so that
pin 2 of the first DTE connects to pin 3 of the second DTE
and pin 2 of the second DTE connects to pin 3 of the first.
These two pins are the most important.
• A null modem is an EIA-232 interface that completes the
necessary circuits to fool the DTEs at either end into
believing that they have DCEs and a network between
them.
• Because its purpose is to make connection, a null modem
can be either a length of cable or a device, or you can make
one yourself using EIA-232 cable and a breakout box that
allows you to cross-connect wires in any way you desire. Of
these options, the cable is the most commonly used and
the most convenient. The cable is also called as cross cable.
Other difference
• Whereas an EIA-232 DTE-DCE interface cable has
female connectors at the DTE end and male
connector at the DCE end, a null modem has
female connectors at both ends to allow it to
connect to the EIA-232 DTE ports, which are
male.
Pin Connection With and Without DCEs
Null Modem